This application relates generally to spinal fusion devices and methods and, more particularly, to spinal fusion devices and methods employing direct current (DC) bone growth stimulation as an adjunct to interbody fusion.
Approximately 100,000 lumbar spine fusion surgeries are performed annually in the U.S. on patients with lower back pain due to degenerative disc disease or spondylolisthesis. Adjacent vertebrae are fused together in such surgical procedures, thereby eliminating motion among the spine levels and significantly reducing lower back pain in most patients. Known spinal fusion methods include posterolateral fusion, also known as inter-transverse process fusion because it involves fusion of the transverse processes of adjacent vertebrae, and interbody fusion, which involves fusion of the bodies of adjacent vertebrae. Traditional interbody fusion involves insertion of autograft or allograft (bone graft from the patient's own body or from another body, respectively) into the intervertebral disc space as a spacer.
Interbody fusion is gaining popularity as an alternative to posterolateral fusion. The introduction of threaded titanium interbody fusion cages has made this technique simpler and more common among orthopedists and neurosurgeons. Such a cage is typically a rigid, hollow cylinder with a pattern of holes through its sidewall to allow bone growth into and through the cage. The cage is typically filled with morselized bone to facilitate fusion by bone growth through the cage, which thereby promotes interbody fusion. A number of interbody fusion devices and techniques have been proposed, as exemplified by those described in the following patents:
Patent No. Inventor Issue Date 4,501,269 Bagby February 26, 1985 4,961,740 Ray October 9, 1990 5,015,247 Michelson May 14, 1991 5,489,307 Kuslich et al. February 6, 1996 5,489,308 Kuslich et al. February 6, 1996
Fusion success rates greater than 85% at 1-2 year follow-ups have been reported for titanium interbody fusion cages in one-level fusions, whereas the success rates for such cages in two-level fusions have been reported to be approximately 70-80%. Fusion success rates as low as 70% present a serious and expensive problem, since a large portion of patients require revision surgery, which is costly, painful, and disabling. One proposed solution to the problem is to pack interbody fusion cages with synthetic bone growth factors, particularly bone morphogenic proteins, which exhibit the ability to induce bone and/or cartilage formation. There is a safety concern over the potential formation of ectopic bone on or near the spinal cord with the use of potent growth factor products. It is also believed that bone growth factors remain at the fusion site for only hours or at most a few days before diffusing away.
Implantable direct current bone growth stimulation has been established as a useful adjunct for posterolateral fusion as well as for the traditional type of interbody fusion involving bone graft alone as the interbody spacer. Spinal fusion stimulators with titanium cathodes are available from Electro-Biology, Inc., the assignee of the present invention, and have been employed with the titanium cathodes placed in contact with bone graft material and the decorticated spine to deliver direct current to the fusion site. A small direct current (typically 20 or 40 .mu.A divided among multiple cathodes) is delivered constantly for six months to enhance bone fusion. However, physicians are cautioned not to allow any metallic part of the stimulator to contact any metal internal fixation device. Bone graft is packed in and around the cathodes to form a fusion mass in which the cathodes are completely embedded.
In spite of all known adjuncts for interbody fusion, there remains a need for devices and methods for speeding interbody fusion and increasing fusion success rates.